Simple buck switching regulator circuit

Edit: Just to add, this is to go into an application that will be running 24x7x365, with around 0.05A current draw for 99% of the time.

I'm wanting a cheap, safe and reliable way of taking AC power into my project box to power a 5V 0.4A (peak current) circuit. After looking at all the options it would seem the best way would be for me to have the following:

Isolation transformer -> Rectifier -> Smoothing cap -> Buck Switching Regulator

Which is certainly within the realms of possible. Although I've got a couple of questions:

1) If a transformer outputs 18VAC at 178mA (supplying power of 3.2VA), would the regulator be able to source 0.64A from it (5V / 3.2VA)... Obviously this is ignoring the voltage drops over the other components.

2) There is a very large price difference between 4 power diodes and a pre-made bridge rectifier, does a pre-made bridge rectifier have any benefit beyond the 4 diodes?

3) I don't need to go down this route because I'm requiring isolation for safety (hence the transformer), and in fairness I'm rather ignorant about flybacks etc. but could I not get an AC/DC converter (from this category on Farnell), which would replace the isolation transformer and rectifier then continue the schematic as above?

Thanks.

Have you considered a 5VDC output, regulated plugpack, (wall wart). You can get them in 1A very easily, and it would be cheaper, smaller and easier than what you're considering. It fits all of your criteria:-

cheap, safe and reliable

I would use a USB adaptor, lots on eBay. ex: 5 for $8.00 ;) http://www.ebay.ca/itm/5-Pack-Micro-USB-Wall-Charger-Adapter-2Amp-Power-For-Samsung-Galaxy-Note-3-S5-/152020749533?hash=item2365248cdd:g:P~cAAOSwAuNW7L8Z

OldSteve: Have you considered a 5VDC output, regulated plugpack, (wall wart). You can get them in 1A very easily, and it would be cheaper, smaller and easier than what you're considering. It fits all of your criteria:-

Yes, I've had a look at them and have considered using them, breaking it open to remove the prongs and fitting it inside the enclosures and filtering the output (200mv p-p appears quite common). They are certainly quite cheap and they do meet the safety standards (assuming I get a branded one), but I'm concerned that at that price point getting one that is reliable (i.e. using well know and reputable component manufacturers) would be very difficult.

LarryD: I would use a USB adaptor, lots on eBay. ex: 5 for $8.00 ;)

Hehe, thank you, no kidding! There certainly are lots but I wouldn't trust anything like them for all the tea in china! :D

1) If a transformer outputs 18VAC at 178mA (supplying power of 3.2VA), would the regulator be able to source 0.64A from it (5V / 3.2VA)... Obviously this is ignoring the voltage drops over the other components.

You should get close to that. You'll have to check the efficiency of the buck regulator. i.e. If it's 90% efficient, you have to feed-in 3.56 VA to get 3.2VA out.

If a transformer outputs 18VAC

Don't forget that your filter capacitor will charge-up to the peak voltage, so you'll get about 25VDC. Measure the voltage. You might get a little more than 18V with no load, and your power line voltage isn't "perfect" or perfectly constant. So, make sure the switching regulator can handle more than 25V input. (And, there is a 1.4V drop across your diode bridge.)

2) There is a very large price difference between 4 power diodes and a pre-made bridge rectifier, does a pre-made bridge rectifier have any benefit beyond the 4 diodes?

It's just convenience. I haven't used 4 separate diodes for many years.

3) I don't need to go down this route because I'm requiring isolation for safety (hence the transformer), and in fairness I'm rather ignorant about flybacks etc. but could I not get an AC/DC converter (from this category on Farnell), which would replace the isolation transformer and rectifier then continue the schematic as above?

Your approach is reasonable. I've never built a full-switching regulator and I wouldn't attempt it unless I had a really-really good reason. It's not an easy thing to build, and when working with line voltage, the odds frying components during testing/debugging is fairly high. Then, there's the chance of something going terribly wrong and feeding line-voltage into my low-voltage circuits.

I've made lots of linear power supplies over the years with a transformer, bridge diode, capacitor, and LM78xx and/or LM79xx regulators. But at 400mA, you'd be "pushing it", especially if you are dropping lots of voltage across the (inefficient) linear regulator.

For my most recent project, I decided to buy a switching supply similar to [u]this[/u]. A power supply like that is more expensive than a wall wart, but I didn't want to use one.